The present invention relates to a process and apparatus for reducing the danger of ignition and explosion from the decomposition of industrial process gases under high pressure, such as ethylene in a high-pressure polymerization plant.
It is well known that there is a danger of explosion in the presence of unstable industrial process gases maintained under pressure that are subject to decomposition. For example, it is well known that in the course of polymerizing ethylene at high pressure (approximately 300 to 3,000 bars) and at high temperature (approximately 150.degree. C. to 350.degree. C.), some operational difficulties such as mechanical failures or insufficient purity in the gaseous ethylene, despite careful monitoring of the pressure and temperature indicators, may result in heating a fraction, even if small, of the ethylene contained in the polymerization reactor or in the separator (operating ordinarily at a pressure between 100 and 500 bars) to a temperature exceeding about 450.degree. C. Such heating is enough to initiate the decomposition of that fraction of ethylene into a mixture of carbon, hydrogen, and methane. Furthermore, the above-cited operating conditions for the reactor and the separator are such as to allow a rapid propagation of any initiated decomposition, invariably resulting in rapid increases in pressure and/or temperature. The reactor and separator are protected against excessive pressure by the rupture of at least one member (disk, valve, relief vent) with a safety function, whereby the decomposition products can escape into the atmosphere. Besides the polluting effect of the expulsion into the atmosphere of the pulverulent carbon, the ignition of the decomposition gases must especially be feared, because it causes violent explosions capable of material damage and human injury.
Several solutions to this problem of deleterious ignition and its consequences, seeking to meet safety requirements in case of decompression of high-pressure polymerization plants, already have been proposed, in particular by U.S. Pat. Nos. 3,781,256, 3,871,458, and 4,115,638, and by the Japanese patent applications 48-51.336/73 and 48-51.337/73 filed on May 9, 1973. All these solutions have in common that they remedy at least one of the three presumed conditions for the ignition of decomposition gases: high pressure, high temperature, and supersonic velocity of the gases. Therefore these solutions generally consist in cooling by various means the decomposition gases of which the initial temperature--as noted above--exceeds 450.degree. C., and may in fact reach 1,500.degree. C. Besides, in order to prevent polluting the ambient atmosphere by the decomposition gases, the recommendation is made to lower the pressure and/or the velocity of the gases below the speed of sound. In short, all these solutions are based on the hypothesis stated in the U.S. Pat. No. 3,781,256, namely that the problem of the ignition of the decomposition gases is reduced to the problem of auto-ignition of these gases due to their own high temperature.
Now the applicants have unexpectedly discovered that contrary to the teachings of the prior art, the auto-ignition of the decomposition gases due to their own high temperature is not necessarily the main cause of ignition in these gases and most of the time is only a secondary cause. This discovery results from experiments carried out on ethylene in the absence of decomposition in a reactor using rupture tests of at least one safety means, the ethylene temperature at the time immediately preceding the rupture not exceeding 200.degree. C. These experiments have shown that despite the absence of anomalous heating prior to rupture of the safety means, an emission of flaming gas at the discharge of the evacuation pipe (chimney) and the presence of a shock wave, which propagates at a rate of approximately 500 to 700 m/s, are noted.
The observed phenomenon might be explained as follows. After the safety means has been ruptured, the air contained in the evacuation pipe (chimney) and initially at rest is passed by a pressure wave moving at a velocity that depends on its intensity and exceeds that of sound in the medium. This pressure wave therefore precedes the flow of the ethylene or of the decomposition gases in the evacuation pipe, and compresses and heats the air contained in the pipe. Further, since in general the evacuation pipe is not wholly linear but comprises at least either a curved section joining the side wall of the reactor or separator to the vertical section of the chimney or a variation in cross-section, the pressure wave as a rule will not be planar and therefore can be reflected from the walls of the evacuation pipe. These wave reflections permit wave focusing on the axis of symmetry of the pipe and therefore the heating of particular point locations in the evacuation pipe. Lastly, the possibility of successive reflections of the safety disk onto the walls of the chimney represents a third source of local heating together with that from the pressure wave and combines its effects with the others.
The heating phenomena described above suffice, even in the absence of any ethylene decomposition, to raise the temperature at particular points in the chimney to above 600.degree. C. The diffusion phenomena, the differences in gas flow speeds in the chimney between its walls and its axis of symmetry, the variations in cross-section, and the changes in the direction of the stack contribute locally to form pre-mixing zones of air and gas. Ignition is initiated at the hot-air/ethylene interface, which moves at a speed less than that of the pressure wave and therefore lags this wave, and more precisely at the level of these pre-mixing zones. The pre-mixing zones thereafter are carried by the evacuation to the outside of the chimney; therefore they disappear rapidly from the chimney when the air is replaced by the gas. Similarly the flame is carried by the flow toward the exhaust section of the chimney, where it remains during the entire period of evacuation. Gas ignition by the pressure-wave effect as just described is enhanced by a high temperature of the gas, whereby the temperature of the air-gas mixture is increased, and consequently a decomposition gas is more likely to ignite by the pressure-wave effect than ethylene at 200.degree. C. This increase in the temperature of the air-gas mixture thus demonstrates that auto-ignition of decomposition gases caused by their own temperature--previously considered to be the root cause of ignition--instead is a derived and secondary effect from the pressure wave.
All the previous art solutions for the ignition problem that are described in the above-cited patents sought to cool the decomposition gases either when being evacuated into the atmosphere or when being recovered. These solutions therefore failed to adequately take into account the time-parameter of the mechanism from the opening of the safety means to the end of the evacuation. This parameter, however, was found to be of crucial importance as shown by the work of applicants. The cited patents state that the duration of evacuation as a rule is between 3 and 10 seconds, and U.S. Pat. No. 3,781,256 states that the time between opening the safety means and the arrival of the gas at the cooling system is about 50 to 100 milliseconds. Under these conditions, it is not surprising that the effectiveness of the prior art solutions should have been inadequate, because, in view of applicants' observations, the duration of the non-steady-state flow phase resulting in the pressure wave effect generally is equal to or less than 25 milliseconds. Considering the discovery of the nature and the duration of the main cause of the ignition of the decomposition of gases under high pressure, such as ethylene in high-pressure polymerization equipment, the effectiveness of a process and apparatus in reducing the risk depends less on the magnitude of the implementing means than on the time of implementation. In particular, it should be noted that the recovery process of the decomposition products described in U.S. Pat. No. 4,115,638 is of a highly uncertain effectiveness when the chances of ignition are not eliminated at the discharge of the reactor or separator.
Furthermore, the prior art solutions did not take into account that the flow rate of the decomposition gases from the pressurized vessel varies with time during the evacuation, during and after its non-steady-state phase. As shown in applicants's research, this phenomenon exerts a crucial influence on the selection of the optimal method for cooling the decomposition gases, as shown further below.